Carlos Barria/Reuters

Marco Rubio and Ted Cruz reminded the audience at Thursday night’s GOP debate about the party’s original big issue in this campaign: immigration.

Touting his disavowal of the Gang of Eight bill, a bipartisan piece of legislation for comprehensive immigration reform, Cruz went on the attack against Rubio, one of the co-authors of the bill.

“Ask people like Jeff Sessions and Steve King and Rush Limbaugh and Mark Levin, all of whom say—as Jeff Sessions said, responding to these false attack attacks just recently in Alabama—he said if it wasn't for Ted Cruz, the Gang of Eight Rubio/Schumer bill would have passed. But because Ted stood up and helped lead the effort, millions rose up to kill it,” Cruz said, listing some of his biggest fans and referring to himself in the third person.

Rubio wasn’t having it and called Cruz a fake conservative.

“This is the lie that Ted's campaign is built on and Rand [Paul] touched upon it—that he's the most conservative guy and everyone else is, you know, everyone else is a RINO [Republican in Name Only],” Rubio said.

“The truth is, Ted, throughout this campaign you've been willing to do or say anything to get votes. You worked for George Bush's campaign. You helped design George W. Bush's immigration policy. When you got to the Senate, you did an interview with CBS News … you said on the issue of people here illegally, ‘We can reach a compromise.’”

And of course, Rubio had to refer to Cruz’s main rival in the race, notably absent from the room: Donald Trump.

“Now you want to trump Trump on immigration,” Rubio said. “You can't—we're not going to beat Hillary Clinton with someone who's willing to say or do anything to win an election.”

Cruz has come under fire from fellow Republicans for waffling on immigration when it was politically convenient over the years. And he’s gone on the offensive recently against Rubio, launching an ad campaign that accuses the the Florida senator of “betray[ing] our trust” for helping to author the Gang of Eight bill.

Rubio, hoping to bring the issue of immigration back to the fore in a debate that included a surprising amount of substantive talk, also tussled with fellow Floridian Jeb Bush, despite having largely similar views on the issue for many years.

Fox News moderator Megyn Kelly challenged Rubio for opposing citizenship for undocumented immigrants during his 2010 Senate campaign, before eventually sponsoring the 'Gang of Eight' immigration bill after he joined the Senate. The Gang of Eight immigration reform package included a pathway to citizenship. Rubio stumbled at her question, saying that he only opposed "blanket amnesty."

Bush, who supports a legal status for undocumented immigrants, said Rubio abandoned the package as soon as he felt it was becoming unpopular. It was an odd exchange between two Republican candidates who both, at one time or another, supported a legal status for immigrants who entered the country illegally.

"You shouldn't have cut and run," Bush told Rubio, though the two have had similar positions on immigration over the years. "Now it's harder and harder to actually solve this problem."

"You are not going to be able to ram down the throat of the American people your approach," Rubio responded.

Without the specter of Trump, the man against whom the other candidates could direct their attacks, it seemed that Rubio and Cruz were fishing for a fight. And they ended up taking it back to where it all began.